High pressure discharge device having a fill including iodine mercury and at least one rare earth metal



Aug. 1, 196 K. H. BUTLER ETAL 3,

HIGH PRESSURE DISCHARGE DEVICE HAVING A FILL INCLUDING IODINE MERCURY AND AT LEAST ONE RARE EARTH METAL Filed 0012. 24, 1965 YTTRIUM LANTHANUM CERI UM NEODYMIUM LUTETIUM HOLMIUM THULIUM PRASEODYMIUM 'GADOLINIUM TERBIUM DYSPROSIUM ERBIUIVI MAY CONTAIN THORiUM SLIVER JOHN F WAYMOUTH' FREDERIC KOURY KEITH H. BUTLER [NVENTORS swa /7;?

ATTORNEY United States Patent HIGH PRESSURE DISCHARGE DEVICE HAVING A FILL INCLUDING IODINE MERCURY AND AT LEAST ONE RARE EARTH METAL Keith H. Butler, Marblehead, Frederic Koury, Lexington, and John F. Waymouth, Essex County, Mass., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Oct. 24, 1965, Ser. No. 504,753 3 Claims. (Cl. 313-229) This application is a contiuation-in-part of our copending application Ser. No. 239,272, filed Nov. 21, 1962, entitled, Electric Discharge Device, now abandoned.

This invention relates to high pressure electric discharge devices and particularly to the production of a white light from such devices.

High pressure electric discharge devices are well known to the art, however white light has never been obtained from their practical commercial embodiments. Most commercially available high pressure electric discharge devices contain mercury vapor and the discharge produced is only the typical mercury discharge consisting of discrete, separate wavelengths, generally called lines. A1- most all of the radiation is contained in the blue region together with a line in the green and a line in the yellow. Hence when a conventional high-pressure mercury discharge device illuminates a red object, particularly one reflecting light only in the range of 6000 to 6800 A., the object appears black.

Attempts have been made by the art to obviate this problem and among the suggested modifications have been the inclusion in the mercury arc stream of various metallic elements which emit radiation at wavelengths different than mercury emission, so that lamps having such inclusions will produce radiations resulting from a combination of mercury lines together with lines of the metallic additions. Typical of the inclusions have been the addition of thallium, zinc, cadmium, or sodium metals. Although such additions did add other lines to the spectrum and improve the color rendition, still only a limited series of separate and discrete lines were generally present. A white light derived from a substantially continuous spectrum was yet to be obtained from such devices.

Another suggested modification was the color-improved mercury vapor lamps wherein the inner surface of the outer bulbous envelopes were coated with fluorescent phosphors which converted some of the invisible ultraviolet arc radiation into vissible light of a red orange color. Although the mixture of the red-orange together with the blue-green of the mercury produced some change in color, there was a loss of efiiciency in the lamp because the phosphor absorbed some of the visible radiation from the arc. Furthermore, phosphor coatings fail to solve the real problem which is the lack of a continuous spectrum in the radiation of the discharge itself.

A white-light or substantially continuous spectrum can be obtained from radiation having a large number of lines very close together and extending throughout the whole visible spectrum, but such radiation has not been present in commercially available electric discharge devices, the spectrum of the latter consisting instead of a few widely separated lines. Quite unexpectedly we have discovered that a continuum is formed when certain metals, which are among the rare earth metals and those closely related therewith, are used in a mercury arc, singly or in combination with each other. A spectrum whose lines are so closely spaced as to be almost a continuum is emitted, and for convenience we shall call such a spectrum a continuum. The rare earth or closely related metals which produce such elfects in an arc tube are lanthanum, cerium, neodymium, thulium, lutetium, holmium, praseodymium, gadolinium, terbium, dysprosium, erbium, and yttrium. Notably, the inclusion of samarium, europium or ytterbium, which are normally considered to be rare earth or closely related metals, results in what we believe to be an inoperative lamp. In addition, mercury either as a metal or as an iodide, must be included in the filling of the tube, the quantity of mercury being such as to allow complete vaporization at normal operating temperatures of the arc tube and to form a restricted arc therein. To achieve maximum white light emission, certain ratios in the number of iodine atoms to mercury atoms must be maintained and although we prefer a ratio of about 0.45, it is possible to use a ratio between 0.10 to 0.85. By a white light or continuous spectrum we mean, as previously noted, an almost complete forest of spectral lines in the emission spectrum, substantially a continuum, appearing generally less than 5 A. apart and containing, superimposed upon this forest, the typical mercury lines at 4048, 4348, 5461, 5770 and 5790 angst-rom units. The measurements of the spectrum can be obtained by measuring the density of photographic records of the spectrum obtained at various exposure times from lamps of varying ratios of iodine to mercury.

Accordingly, the primary object of this invention is production of a white light from a high pressure discharge device.

A feature of this invention is the incorporation of mercury either as the metal or as the halide together with controlled quantities of the certain rare earth and closely related metals listed above.

A further feature of this invention is the discovery that the thorium sliver normally inserted in the electrodes may be eliminated, if desired.

Many other objects, features and advantages of the present invention will become manifest to those conversant with the art upon making reference to the detailed description which follows and the accompanying sheet of drawing in which preferred embodiments of an electric discharge device which emits white light are shown and described and wherein the principles of the present invention are contained by illustrative examples. Of these drawmgs:

The figure is an elevational view of a high pressure electric discharge device illustrating the positioning of an electric discharge are tube disposed within an outer bulbous envelope.

According to our invention we have discovered that an arc tube having a filling of a combination of certain materials can produce a continuum without the necessity of a correctional phosphor, the lamp having an efficiency of to lumens per watt. In particular, we have discovered that when mercury .and iodine, added either as the individual elements or as the com-pound, in very specific atomic ratios of iodine to mercury are included in the arc tube together with specific quantities of certain rare earth metals or mixtures thereof, that white light can be obtained. It is sometimes advantageous to add 5.25 l0 to 68x10" gram atoms per centimeter of arc tube length of a material, such as sodium, forming a low vapor pressure iodide, to stabilize the arc and insure at room temperature that there is little or no iodine in the vapor state thereby lessening starting voltages.

The lamps according to our invention have an efficiency which is considerably higher than conventional mercury lamps; 70 to 90 lumens per watt and even higher being attainable. The reason for the large increaseis that approximately 50% of the total energy of the rare earth metal spectrum is concenerated in the visible range whereas with conventional mercury lamps only 23% is in the visible. In addition, the use of mercury, iodine and certain rare earth metals in the arc tube fill allows the inclusion of other metals in the system so as to allow for modification of the arc color. Such inclusions can be easily used to modify the emission color of the lamp. For example, cadmium may be added for increased red or thallium for increased green. However, in the case of cadmium, the inclusion adds to the cadmium spectrum, suppressing the mercury spectrum slightly but not affecting the general configuration of the rare earth metal spectrum. On the other hand, the inclusion of thallium improves the efficiency because of the elements characteristic strong green line.

Referring to the figure, an elevational view of a high pressure electric discharge device is shown. The device such as shown in the drawing comprises an outer vitreous envelope or jacket 2 of generally tubular form having a central bulbous portion 3. The jacket is provided at its end with a re-entrant stem having a press through which extend relatively stiff lead-in wires 6 and 7 connected at their outer ends to the electrical contacts of the usual screw-type base 8 and at their inner ends to the arc tube and the harness.

The are tube is generally made of quartz although other types of glass may be used such as alumina glass or Vycor, the latter being a glass of substantially pure silica. Sealed in the arc tube 12 at the opposite ends thereof are main discharge electrodes 15 and 16 which are supported on lead-in wires 4 and 5 respectively. Each main electrode 15 and 16 comprises a core portion which may be a prolongation of the lead-in wires 4 and 5 and may be prepared of a suitable metal such as for example molybdenum or tungsten. The prolongations of these lead-in wires 4 and 5 are surrounded by molybdenum or tungsten wire helixes.

An auxiliary starting probe or electrode 18, generally prepared of tantalum or tungsten is provided at the base end of the arc tube 12 adjacent the main electrode 14 and comprises an inwardly projecting end of another leadin wire.

Each of the current lead-in wires described have their ends welded to intermediate foil section of molybdenum which are hermetically sealed within the pinched sealed portions of the arc tube. The foil sections are very thin, for example approximately 0.0008 inch thick and go into tension without rupturing or scaling off when the heated arc tube cools. Relatively short molybdenum Wires 23, 24 and 35 are welded in the outer ends of the foil and serve to convey current to the various electrodes inside the arc tube 12.

Metal strips 45 and 46 are welded into the lead-in wires 23 and 24 respectively. A resistor 26- is welded to foil strip 45 which in turn is welded to the arc tube harness. The resistor may have a value of for example, 40,000 ohms and serves to limit current to auxiliary electrode 18 during normal starting of the lamp. Metal foil strip 46 is welded directly to stiff lead-in wire 7. Lead-in Wire 35 is welded at one end to a piece of molybdenum foil sealed in the arc tube 12 which in turn is welded to main electrode 13. Metal foil strip 47 is welded to One end of lead-in 35 and at the other end to the harness. The pinched or flattened end portions of the arc tube 12 form a seal which can be of any desired width and can be made by flattening or compressing the ends of the arc tube 12 while they are heated.

The are tube 12 is provided with a filling of mercury in a quantity such as to vaporize completely when a pressure in the order of one half to several atmospheres is reached during normal lamp operation at temperatures of 450 to 700 C. Particularly we have found that through the addition of certain mercury iodides to the mercury fill, that the quantity of the latter can be reduced to as low as 1.25 X10" gram atoms per centimeter of arc length and possibly lower; arc length being measured as the distance between opposing tips of the main electrodes 15 and 16. The amount of mercury added can be varied widely as we have indicated, although we prefer to add approximately 2.5 X10 to 4.l l gram atoms of mercury per centimeter of arc length. The continuum or white light emission appears to be substantially independent of the amount of mercury metal added and hence the quantity of the latter can be reduced while white light emission is still attained.

As is conventional in the art, a quantity of rare gas such as helium, argon, neon, krypton or xenon at a pressure of about 25 millimeters of mercury is added to facilitate starting. In addition to these materials we have discovered that a mercury iodide, or combination of elemental mercury and elemental iodine, must be added to the arc tube to attain white light emission. Particularly we have discovered that about 0.45 atom of iodine must be added for every atom of mercury irrespective of whether the mercury atoms are derived from the metal or from the mercury compound, although this ratio may be varied in reasonable tolerances between 0.10 to 0.85 atom of iodine per atom of mercury. It is quite important to use anhydrous material in the arc tube since the incorporation of water tends to make the discharge hard to start.

The U-shaped internal wire supporting assembly or are tube harness serves to maintain the position of the arc tube 12 substantially coaxial within the envelope 2. To support the arc tube 12 within the envelope, stiff lead-in wire 6 is welded to the base 53 of the harness. Because stiff lead-in wires 6 and 7 are connected to opposite sides of a power line, they must be insulated from each other together with all members associated with each of them. Clamps 56 and 57 hold the arc tube 12 at the end portions and fixedly attached to legs 54 of the harness. A rod 59 bridges the free ends of the U-shaped support wire 54 and is fixedly attached thereto for imparting stability to the structure. The free ends of the U-shaped wire 54 are also provided with a pair of metal leaf springs 60, frictionally engaging the upper tubular portion of the lamp envelope 2. A heat shield 61 is disposed beneath the arc tube 12 and above the resistor 26 so as to protect the resistor from excessive heat generated during lamp operation.

As we have stated, for attaining maximum white light emmission a ratio of iodine atoms to mercury atoms should be maintained at approximately 0.45. However, white light emission will still be obtained when the ratio is greater or less than 0.45 (within definite limits) but the emission is reduced using such off peak ratios. Since it is difficult if not impossible to produce lamps in production lines wherein the ratio of halogen to mercury is exactly 0.45, tolerances are allowed between 0.10 to 0.85 and within such tolerances, reasonable white light emission is still evidenced.

When adding the rare earth metal, 0.046 l0*' to 1.1 10 gram atoms per centimeter of arc length should be added. Above this range the rare earth metals tend to deposit upon the Walls of the arc tube, while below the range no continuous spectrum is evidenced. The rare earth metals may be added as the metals per se, or as the corresponding iodides or as mixtures thereof. Quite surprisingly, we have discovered that all of the rare earth metals do not work for our purposes and only these previously recited are applicable.

The fabrication of the envelope, sealing technique and positioning of the electrodes in the high pressure electric discharge device according to our invention takes place in a manner quite similar to that known to the art with conventional mercury lamps. And further, the mercury metal may be added to the arc tube by techniques well known to the art. To prepare the arc tube, we pump down an envelope having a pair of electrodes disposed at either end thereof, through an exhaust tubulation extending from the surface of the envelope and disposed in communication with the interior thereof. The envelope is then heated with a torch and filled with argon to flush out residual impurities. It is quite important to eliminate or substantially eliminate the presence of hydrogen from the are tube. Hydrogen is known to affect the starting oi mercury lamps adversely but its effect appears to be greater in the lamps prepared according to our invention. The difiiculty with hydogen appears to be due to the formation of hydrogen iodide which has a much higher vapor pressure than any other iodide present. We believe that for every atom of hydrogen present, that there is an extra atom of iodine in the vapor state. Presence of the iodine in the vapor state increases the voltage which must be ap plied to the lamps for starting. Hence, not only must hydrogen be substantially eliminated from the gases in the fill tube but each and every part going into making up the arc tube must be freed of residual hydrogen impurities. For example, the electrodes should be vacuum baked at 300 to 400 C. for a few hours before their use to eliminate hydrogen which might occur due to processing. Furthermore, care should be exercised when sealing the electrodes into the arc tube to prevent hydrogen-containing, combustion gases from seeping in or becoming absorbed upon the surface.

The pump and fill procedure above described is usually repeated three to four times and then an arc is struck between the electrodes while there is a filling of argon gas. This operation of the arc removes any residual impurities from the electrodes and these contaminates can then be easily drawn from the system when the argon filling is pumped out. We then add approximately 34 milligrams of mercury, 13 milligrams of iodine and 5.0 milligrams of a rare earth metal such as cerium to an envelope having an arc length of approximately 7.5 centimeters. The arc tube is then filled to atmospheric pressure with argon gas which is slowly leaked out until a pressure of about 23 millimeters of mercury is obtained. Subsequently, the exhaust tubulation is tipped oil and the envelope is sealed. Testing of the lamp indicates that a white light emission is evidenced, which emission is in the order of 70 lumens per watt.

Exemplary of some of the other metals which can be added are the following together with the quantities which may be added to a 7.5 centimeter arc tube. The percent of red column is a measure of the improvement upon standard mercury lamps which can be attained through the addition of rare earth and closely related metals. Mercury respectively being present therein at an atomic ratio of has very few lines from which all of the light of the lamp comes, whereas the metals according to our invention contribute to the continuum.

TABLE I Mgs. Mgs. Mgs. Percent Hg I Metal red Conventional high pressure mercury 1am ns 68 Negligible Metal:

Lanthanum 84 13 5 3. 1 Cerium 34 13 5 2. 2 Praseodymium 34 13 5 3. 0 N eody-mium 34 13 5 3. 3 Gadolinium..-" 34 13 5 2. 7 Terbium 34 13 5 2. 8 Dysprosium. 34 18 5 3. 1 Holmium" 34 13 5 3. 1 Erbium 34 13 5 3. 7 Thulium 43 1 18. 9 5 l. 3 Yttrium. 34 13 5 1. 9 Lutetium 2 "I 63 1 15 4 4. 0

1 Mg. H I3 2 4.5 cm. are tube.

0.10 to 0.85 and said mercury being present in sufiicient quantities to be completely vaporized at predetermined operating temperatures of said are tube and to form a restricted arc therein, said rare earth metals being present in quantities of 0.046 10- to 1.1 10 gram atoms per centimeter of arc length.

2. A high pressure discharge device comprising an arc tube having electrodes sealed at either end thereof and a vaporizable fill of a quantity of iodine, mercury, sodium and at least one rare earth metal selected from the group consisting of yttrium, lanthanum, lutetium, holmiu-m, thuliurn, cerium, neodymium, praseodymium, gadolinium, terbium, dysprosium and erbium, said iodine and mercury respectively being present therein at an atomic ratio of 0.10 to 0.85 and said mercury being present in sufiicient quantities to be completely vaporized at predetermined operating temperatures of said are tube and to form a restricted arc therein, said rare earth metal being present in quantities of 0.046 10 to 1.1)(10 gram atoms per centimeter of arc length.

3. A high pressure discharge device comprising an arc tube having electrodes sealed at either end thereof and a vaporizable fill of a quantity of iodine, mercury, sodium and at least one rare earth metal selected from the group consisting of yttrium, lanthanum, cerium, neodymium, praseodymium, gadolinium, terbium, dysprosium, lutetium, holmium, thulium, and erbium said iodine and mercury being present therein in sufiicient quantities to be completely vaporized at predetermined operating temperatures of said are tube and to form a restricted arc therein, said rare earth metal being present in quantities of 0.046 10 to 1.1 10 gram atoms per centimeter of arc length and said sodium being present in quantities 5.25 10' to 6.8 10- gram atoms per centimeter of arc length.

References Cited UNITED STATES PATENTS 3,153,169 10/1964 Bauer 313--228 3,234,421 2/1966 Reiling 313-25 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

1. A HIGH PRESSURE DISCHARGE DEVICE COMPRISING: AN ARC TUBE HAVING ELECTRODES SEALED AT EITHER END THEREOF AND A VAPORIZABLE FILL OF A QUANTITY OF IODINE, MERCURY AND AT LEAST ONE RARE EARTH METAL SELECTED FROM THE GROUP CONSISTING OF YTTRIUM, LANTHANUM, CERIUM, NEODYMIUM, LUTETIUM, HOLMIUM, THULIUM, PRASEODYMIUM, GADOLINIUM, TERBIUM, DYSPROSIUM AND ERBIUM, SAID IODINE AND MERCURY RESPECTIVELY BEING PRESENT THEREIN AT AN ATOMIC RATIO OF 0.10 TO 0.85 AND SAID MERCURY BEING PRESENT IN SUFFICIENT QUANTITIES TO BE COMPLETELY VAPORIZED AT PREDETERMINED OPERATING TEMPERATURES OF SAID ARE TUBE AND TO FORM A RESTRICTED ARC THEREIN, SAID RARE EARTH METALS BEING PRESENT IN QUANTITIES OF 0.046X10-5 GRAM ATOMS PER CENTIMETER OF ARC LENGTH. 